Just a note.
I just checked in at Frankfurt airport with China airlines to Bangkok. Since my return flight ist more then 45 away, the airline wanted prove of onward travel. I explained that I'll do a border run or similar. They called Bangkok immigration. I was told that in case the immigration officer at Bangkok airport wants to see a ticket, I can be refused entry. They gave me my boarding pass and I was good to go.
But, I booked a flight online to danang Vietnam straight away. Not to take any chances!